A transmission device in a radio communication system is generally provided with a power amplifier that amplifies a transmission signal. When the power of an input signal is relatively small, the power amplifier linearly amplifies the input signal and accordingly the linearity between the input power and the output power can be maintained. On the other hand, when the power of an input signal is relatively large, the saturation area of input/output characteristics is reached and the linearity between the input power and the output power collapses. For this reason, when the power amplifier is caused to run at the saturation area that is generally efficient, a non-linear distortion occurs in the transmission signal that is amplified by the power amplifier. The non-linear distortion can be a cause of an increase in adjacent channel leakage ratio (ACLR) and therefore it is preferable to reduce the non-linear distortion.
The transmission device may employ a distortion compensation technique using the pre-distortion system that compensates the non-linear distortion by adding in advance a distortion having characteristic inverse to those of the non-linear distortion that occurs in the power amplifier. The distortion that is added in advance to the transmission signal in the pre-distortion system is also referred to as a distortion compensation coefficient. For example, a distortion compensation coefficient corresponding to the power level of the transmission signal is read from a lookup table and the transmission signal is multiplied by the distortion compensation coefficient. When the transmission signal that is multiplied by the distortion compensation coefficient is amplified by the power amplifier, a non-linear distortion that occurs in the power amplifier offsets the distortion from the distortion compensation coefficient and accordingly the distortion of the transmission signal is reduced.
Distortions that occur in transmission signals include, in addition to non-linear distortions that occur in power amplifiers, burst distortions that occur in burst signals that are transmitted in time division duplex (TDD) communications. In a burst distortion, the power increases at a rise of a burst signal and the power decreases as the time elapses. Specifically, when transmission of a burst signal is started, the power amplifier starts running and warms up rapidly and accordingly the power of the burst signal increases significantly in few tens of to hundreds of nano seconds (ns) after the start of the transmission. Once few tens of to hundreds of nano seconds after the start of the transmission elapses, the entire power amplifier warms up and the gain decreases gradually and accordingly the power of the burst signal decreases gradually. Similarly to the case of non-linear distortion, examinations have been made on reducing a burst distortion that changes, as described above, the power of a burst signal by adding a correction coefficient having characteristics inverse to the burst distortion to a transmission signal.
Patent Document 1: International Publication Pamphlet No. WO 2008/120318
Patent Document 2: International Publication Pamphlet No. WO 2008/126217
The burst distortion however contains, in addition to an average change that gradually reduces the power from the top to the end of the burst signal, an instantaneous change that changes an instantaneous power of the burst signal in a shorter period. Furthermore, there is a problem in that only reducing the average change corresponding to the elapsed time from the top of the burst signal leaves the instantaneous change and thus it is not possible to correct the burst distortion sufficiently.
In other words, while the average change that gradually reduces the power of the burst signal is observed when the burst signal is viewed macroscopically from the top to the end, the instantaneous change that changes the power of the burst signal instantaneously is observed when the burst signal is viewed microscopically. In order to improve the quality of the burst signal, it is preferable that the burst distortion including not only the average change but also the instantaneous change be corrected.